When several processing units are connected to a time-shared bus of a data-processing network, two main issues or problems have to be considered: the problem of the priority scheme according to which access to the bus is granted to connected devices, and the problem of the continuity of the transmission on the bus when one or several devices are unplugged or malfunctioning.
Such problems appear currently in the field of communication controllers, where a plurality of line-adapters have a time-shared access to a Central Control Unit (CCU) by means of a bus including one or several control lines. Therefore, for the sake of simplicity in the following descriptions, the devices connected in a daisy-chain configuration will be assumed to be the line-adapters of a communication controller, hereafter called "adapters", without prejudice to the generality of the invention. In fact, a similar reasoning would apply to a variety of other devices: processors, memories and the like.
When the adapters connected to a time-shared bus are controlled by a control line which is conventionally daisy-chained, their operation has two main drawbacks:
1. Since the control signal which grants access to the bus is propagated from one adapter to the succeeding one down the bus, the adapters closer to the bus origin have a higher priority level than the ones located farther.
2. When an adapter receives the grant signal and does not request access to the bus for transmission, it has to transmit said grant signal to the succeeding adapter, which may have requested access to the bus. Consequently, when an adapter is unplugged or malfunctioning, the grant signal is not propagated farther, and therefore all the adapters located after the interruption cannot work until the bus continuity has been reestablished.
These two drawbacks have been partly overcome in U.S. Pat. No. 4,380,052, where a plurality of processing units are connected to a single daisy-chained bus assignment control line. This control line is a closed loop, hence giving each processing unit an equal chance to receive the grant signal allowing access to the transmission bus.
Besides, according to cited U.S. patent, each unit is provided with means permitting to detect when the preceding unit is malfunctioning or is turned off, and in response thereto, to regenerate the bus assignment control signal and transmit it to the next succeeding unit if it does not request access to the transmission bus. Thus the continuity of operation of the bus is preserved, provided that two or more adjacent units are not simultaneously failing. Since the means for detection of malfunctioning include a timer, each interruption in the control signal propagation implies a significant loss of time.
However, in case of failure of a unit, a malfunction circuit is activated.
The element failing to receive the pulse to be propagated re-initiates a similar pulse after one timer period, so that the system can continue to operate. Nevertheless, each interruption of the control signal propagation is added to the overall transmission time of said signal on the control line and therefore, it implies a significant loss of time, cutting down the performance of the transmission bus.
Furthermore, the delay introduced by each timer may be very difficult to predetermine, since it must be greater than the longest transmission time of each processing unit, said transmission time depending besides on the task being performed by each unit. Finally, during each transmission cycle of the bus, there will be as many timer durations wasted, as there are non-working adapters in the loop.
It is, then, an object of the present invention to provide a circuit for improving the operation of a plurality of adapters connected to a bus in a daisy-chain configuration, defining the same priority level for each adapter, and allowing one or several adapters to be unplugged or turned off without interruption of the operation of the transmission bus, and without decrease of its performance.